When using a Marketing Cloud Starter/Pro Suite/Foundation Orgs, the Segment Selector is not available within the email preview experience.
If you later upgrade to Marketing Cloud Next Growth or Advanced edition, the Segment Selector may still not appear within the preview. This can occur if your Growth or Advanced edition is still operating on the original Starter Org configuration.
To enable the Segment Selector in the email preview, a migration from the Starter Org to the new Growth or Advanced environment is required.
1. Complete the Migration:
When navigating to the Unified Messaging → Email section in Salesforce Setup, you encounter a Migration page that houses the Transfer to Marketing Cloud feature.
Clicking the Transfer button on the Migration page
Important: Review the official Salesforce documentation to understand the impact of the migration:
Transfer to Marketing Cloud feature on Migration setup page.
2 - Once this is done, you should be able to see the segment selection when doing a preview. The next step would be to make sure that the segment you would like to select are published as saved segment will not be returned by the segment selector.
To publish a segment please follow the steps from this documentation : Publish a segment
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